With the development of ADSL (Asymmetrical Digital Subscriber Line) technology, the related technologies keep being renewed, and evolve from the first generation ADSL to the second generation ADSL2, ADSL2+ and the even newer VDSL2, in the meanwhile, the frequency bands used in various technologies increase gradually, and the corresponding bandwidths also increase gradually. When the frequency spectrum below 1.1 MHz is used in the downstream of ADSL and ADSL2, a maximum downstream rate of 8 Mbps can be provided, and the downstream bandwidth is extended to 2.2 MHz in ADSL2+, with which a maximum downstream rate of 24 Mbps can be provided, and even frequency spectrum up to 30 MHz can be used by the downstream bandwidth of VDSL2, which can provide a symmetric upstream and downstream rates up to 100 Mbps.
However, with the increase of the frequency band used in xDSL technology, crosstalk, especially the crosstalk in the high-frequency range becomes more significant. The crosstalk includes NEXT (Near-End Crosstalk) and FEXT (Far-End Crosstalk), as shown in FIG. 1 and FIG. 2, wherein the NEXT does not have much impact on the performance of the system, however, the FEXT can seriously affect the transmission performance of the line.
For example, when multiple subscribers apply for subscribing to the xDSL service in a bundle of cables, the transmission rate of some lines may be lowered, the transmission performance is unstable, or even the xDSL service can not be provided due to the FEXT, as a result, the line activation rate of DSLAM (Digital Subscriber Line Access Multiplexer) will be definitely and relatively low.
With regard to the crosstalk, at present, in simple cases, some providers have established their own criterions for the application and management of the frequency spectrum to avoid mutual disturbance of the crosstalk among the devices at various locations.
In order to improve the crosstalk issue among multiple subscribers effectively, firstly, a quantitative measurement of the crosstalk existing among multiple subscribers should be executed, after that, the corresponding measures for restraining the crosstalk can be taken according to the obtained amount of crosstalk. However, currently the quantitative test is not available for the crosstalk among multiple lines. Therefore, at present, the performance of a DSL line or system can be optimized only according to some statistical characteristics of the crosstalk, in such case, the corresponding optimization degree of the DSL line is apparently unsatisfactory.